Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of syngas (synthesis gas), being a gas mixture that is rich in both carbon monoxide and hydrogen, with hydrocarbon compounds containing olefinic unsaturation (hereinafter “olefinic material”). The reaction is generally performed in the presence of a hydroformylation catalyst such as cobalt or rhodium, usually in a dissolved or homogeneous form, and results in the formation of a product comprising an aldehyde which has one more carbon atom in its molecular structure than the starting olefinic material. By way of example, higher alcohols useful as intermediates in the manufacture of plasticizers, detergents, solvents, synthetic lubricants, and the like, are produced commercially in the so-called Oxo Process by conversion of C2 or higher olefinic material fractions (typically C5-C12) to an aldehyde-containing oxonation product having one additional carbon atom (e.g., C6-C13). Further treatment of the Oxo product by hydrogenation and distillation yields the respective alcohols. Alternatively the aldehydes may be oxidized to the respective acids.
The Oxo Process to convert olefinic material to aldehydes generally proceeds through three basic stages. These are explained below by specific reference to a cobalt catalyst, but the stages are in general also applicable to processes employing other hydroformylation catalysts.
In the first stage, the hydroformylation or oxonation reaction, the olefinic material and the proper proportions of CO and H2 are reacted in the presence of a homogeneous, typically cobalt-containing hydroformylation catalyst to give a product comprising predominantly aldehydes containing one more carbon atom than the reacted olefin. Typically, alcohols, paraffins, acetals, and other species are also produced in the hydroformylation reaction. The catalytic species, in the example of cobalt, is a carbonyl species containing Co−1 and the catalyst can be supplied to this reaction stage by numerous methods known in the art, such as by injecting an aqueous solution of a cobalt salt (such as cobalt acetate or cobalt formate) directly, or by supplying cobalt from a precarbonylation stage or catalyst makeup stage in the form of a cobalt compound already containing cobalt as Co−1, or by supplying an organically soluble form of Co+2, such as cobalt naphthenate, cobalt oleate, or the cobalt salt formed with heavy acid byproducts of the oxo process, or by supplying cobalt oxides, such as in a slurry. Cobalt that is supplied to the oxonation reaction in a form other than Co−1 is then converted, under the oxonation reaction conditions, to the Co−1 species. This conversion is also called preforming.
The oxonated organic mixture from the oxonation (or oxo) reactor(s), which typically contains various salts and molecular complexes of the metal from the catalyst (i.e., the “metal values”) as well as the aldehydes, alcohols, acetals and other species, and which is conventionally referred to as the “crude oxo product” or “crude hydroformylation mixture”, is treated in a second (demetalling) stage. In the demetalling stage, typically the cobalt (Co−1) species is oxidised to the Co+2 form, and the Co+2 species are then converted to a water soluble salt such as cobalt acetate or cobalt formate. The crude hydroformylation mixture is then separated into phases with the organic phase comprising the desired aldehyde (with as little as possible remaining catalyst), and the aqueous phase comprising the cobalt salt. The organic phase (“crude aldehyde”) is sent to other unit operations downstream, to be converted to the desired final product. For convenience, air is often used as oxidant in the oxidation reaction. The gaseous product mixture produced by the air demetalling process will be a mixture of gases comprising possibly remaining air, hydrogen, carbon monoxide and, depending on their vapour pressure, also some of the reaction products such as aldehydes and alcohols. DE-A-19939491 discloses an oxo process where air is used for the oxidation reaction. The present invention is concerned with improving such a demetalling operation.
In a third stage the metal values removed in the second stage may be worked up in a way that they can be reused in the oxonation (first) stage. There are several ways taught in the prior art to work up this catalyst. For example, one way is to convert the aqueous metal salt to an organically miscible compound such as cobalt naphthenate, and inject it as an organic solution directly into the oxonation reactor(s). Another way is to subject the aqueous salt solution in the presence of an organic solvent to high pressure synthesis gas, converting it to active carbonyl similar to preforming, and delivering it to the oxonation stage via extraction, stripping or the like.
A more complex variation of a catalyst cycle employing an oxidation treatment is disclosed in U.S. Pat. No. 4,404,119. The process in U.S. Pat. No. 4,404,119 performs the air oxidation on an aqueous extract of Co2[Co(CO)4]2 from the hydroformylation reaction product, after first preforming the aqueous solution to maximise the presence of the cobalt carbonyls up to its about 67%. The purpose of the oxidation step is to produce water-insoluble Co2(CO)8. The oxidation is performed optionally in the presence of an organic solvent, and the presence of CO was found to significantly improve the yield of Co2(CO)8. The oxygen-containing gas in the oxidation step of U.S. Pat. No. 4,404,119 is not introduced into the organic reaction product of a homogeneous metal-catalysed reaction.
In U.S. Pat. No. 5,986,145, an aqueous solution of 3-hydroxypropanal is contacted with oxygen to oxidise cobalt or rhodium hydroformylation catalyst species to water soluble species and byproduct carbon monoxide.
In EP 649851, the distillation residue of a hydroformylation reaction mixture containing deactivated rhodium complex catalyst is treated with a mixture of oxygen and carbon monoxide to synthesis the rhodium carbonyl compound, as the first step to synthesize the triorganophosphorus rhodium complex that is used as a catalyst in numerous processes, such as olefin hydroformylation. The carbon monoxide in EP 649851 is present as a reagent to form the rhodium carbonyl. The process of EP 649851 does not involve an aqueous phase to dissolve the oxidised metal residues and therefore is not suitable for demetalling the organic reaction product.
An improvement in the oxo process is taught in U.S. Pat. No. 4,625,067 (“Cobalt Flash Process”). The “Cobalt Flash Process” is defined as a process comprising the recovery of cobalt values from a cobalt carbonyl containing organic stream by contacting this organic stream in a stripper reactor with a stripping gas to entrain volatile cobalt compounds, in the presence of water or preferably an aqueous acid. The cobalt carbonyl containing organic stream may be crude oxo product, but in some variations may also be a cobalt performer product, or a combination thereof. A large portion of the cobalt values dissolved in such streams are in the form of cobalt carbonyl compounds, partly as dicobaltoctacarbonyl (Co2(CO)8) but primarily as hydr(id)ocobalt(tetra)carbonyl (HCo(CO)4). Under stripper-reactor conditions the Co2(CO)8 disproportionates at the oil/water interface to form cobalt anions Co(CO)4− and cobalt cations Co2+. Under acidic conditions the cation forms a cobalt salt, and the anion forms more HCo(CO)4, which may transfer again into the organic phase. The hydrocobaltcarbonyl is fairly volatile and can therefore be taken with the stripping gas overhead in the stripper reactor(s) and returned to the oxo reactor(s) by adsorption into the olefin feed stream. The acidic conditions in the stripper reactor enhance the formation of undissociated hydrocobaltcarbonyl which can be stripped. The partially decobalted crude product is then passed to the demetalling reaction and oxidised and optionally contacted with more aqueous acid as previously discussed. This oxidative demetalling reaction downstream of the stripping step removes most of the remaining cobalt traces from the organic product. This cobalt also ends up as cobalt salt in an aqueous solution, and it may be reused in the hydroformylation step, advantageously together with the cobalt salt solution from the stripper reactor, with methods such as the one set out below.
Numerous improvements and variations on the Cobalt Flash Process have been proposed, such as in U.S. Pat. Nos. 5,235,112; 5,237,104; 5,237,105; 5,336,473; 5,410,090; 5,457,240; WO 93/24437; WO 93/24436, WO 03/082788 and WO 03/082789.
In the Cobalt Flash Process, after the aqueous phase comprising cobalt formate or acetate is separated from the organic phase comprising oxo product in the demetalling reaction, the aqueous phase is passed to an evaporator where cobalt formate or acetate is concentrated before being passed to a preformer, wherein the aqueous cobalt salt (Co+2) is converted to oil soluble Co−1 by reaction with carbon monoxide and hydrogen, in the presence of an oil phase (commonly an aldehyde or alcohol containing stream, such as the product of the oxonation or the downstream hydrogenation reaction). Additional fresh cobalt catalyst is typically necessary and is added as, for instance, cobalt acetate.
U.S. Pat. No. 5,237,105 relates to an improvement in the process of U.S. Pat. No. 4,462,506, in that it provides a method of recovering cobalt values which does not cause the build up and recycle of unreacted light hydrocarbons within the system. It thereby avoids the need for a relative decrease in the net olefinic material feed rate in the case where a rather volatile olefinic feed material is processed. This is accomplished by providing an oxidative demetalling step prior to the stripping step. This produces an almost completely cobalt free organic hydroformylation reaction product and an aqueous product containing most or all of the cobalt as a water soluble cobalt salt. The essentially cobalt free organic phase may then be diverted for further downstream treatment, while the aqueous product containing the water soluble cobalt salt is concentrated, and the cobalt is then converted to cobalt carbonyl for reuse in a performer. The preformer effluent is then stripped to remove volatile cobalt compounds. U.S. Pat. No. 5,237,105 therefore provides a method for removing cobalt values from the crude product of a cobalt-catalyzed hydroformylation reaction formed from an olefinic material such as olefin mixtures having a carbon number in the range C4-C14 wherein an acid-air cobalt demetalling step is provided upstream of the stripping step of a Cobalt Flash hydroformylation catalyst recovery process.
The method of U.S. Pat. No. 5,237,105 comprises the steps of: (a) contacting the crude product with a stream of oxygen-containing gas, an organic acid and water, thereby producing an offgas stream, a substantially cobalt-free organic hydroformylation reaction product and an aqueous product containing water soluble cobalt salt; (b) separating the offgas and the substantially cobalt-free crude product from the aqueous product; (c) diverting the substantially cobalt-free organic hydroformylation reaction product for further downstream treatment such as distillation and/or hydrogenation; (d) concentrating the aqueous product containing the water soluble cobalt salt, thereby producing a concentrated aqueous solution of cobalt salt and a substantially cobalt-free water fraction containing a part of the organic acid, whereby the concentrated aqueous solution of cobalt salt is separated from the substantially cobalt-free water fraction containing a part of the organic acid; (e) recycling the substantially cobalt-free water fraction containing organic acid to step (a); (f) contacting the concentrated aqueous solution of cobalt salt with an alcohol stream and synthesis gas, and passing this mixture to a preforming reactor where a significant portion of the cobalt salt in the concentrated aqueous solution of cobalt salt is converted to a cobalt carbonyl; (g) contacting the preforming reactor effluent containing the cobalt carbonyl with a stream of stripping gas to entrain volatile cobalt compounds in the stripping gas and to generate, as bottoms, alcohol products and remaining dissolved cobalt salts, whereby the entrained volatile cobalt compounds are taken out overhead and the alcohol products and remaining dissolved cobalt salts are taken out as a mixed organic/water bottoms stream; (h) separating the alcohol products of step (g) from the dissolved cobalt salts; (i) recycling the alcohol products from step (h) to step (f); (j) recycling the dissolved cobalt salts from step (h) to step (a); and (k) contacting the volatile cobalt compounds from step (g) with the olefinic material; whereby the volatile cobalt compounds are absorbed into the olefinic material.
In the process of U.S. Pat. No. 5,237,105, it is preferred that the oxygen-containing gas, introduced into the system in step (a) to oxidise the Co−1 species to the Co+2 species, be at least one gas selected from the group consisting of: air, air with nitrogen, carbon dioxide, and mixtures of inert gases with oxygen having an oxygen content in the range of about 2 to about 10% vol. The amount of oxygen-containing gas used in the catalyst removal process of U.S. Pat. No. 5,237,105 is a function of the cobalt contained in the crude oxo product. U.S. Pat. No. 5,237,105 gives an example wherein the oxygen-containing gas is a mixture of air and nitrogen, and the nitrogen is used to dilute the mixture to about 4 volume % of O2, i.e., 4.11 grams of N2/gram of air. Thus, the air and nitrogen mixture is added to the crude oxo product in an amount of approximately 8.81 grams of gas mixture/gram of cobalt. Since the cobalt concentration in commercial crude oxo products is preferably in the range from about 0.05 to about 0.50 weight %, the oxygen-containing gas is typically added to the crude oxo product in an amount of from about 0.45 to about 4.50 weight %. The oxygen-containing gas is then used in a weight ratio of oxygen-containing gas relative to crude oxo product of from about 0.0045:1 to 0.45:1.
Although it is not stated in U.S. Pat. No. 5,237,105, it is believed that the nitrogen diluent is provided for safety reasons. In many hydrocarbon processing reactions and in particular those which involve the flow of hydrocarbon streams, there is a danger of electric discharge occurring inside the process equipment. In systems such as in the cobalt removal processes described in U.S. Pat. No. 4,625,067 or DE-A-19939491, which employ oxygen-containing streams, such as air, in conjunction with the hydrocarbon stream, there is a concern that the streams may combine to form a flammable and/or explosive mixture. Accordingly, in U.S. Pat. No. 5,237,105, large amounts of nitrogen are used to dilute the mixture to about 4 volume % of oxygen to render the gas non-flammable. Even if the oxygen is considered to react very quickly during normal operation, either with the cobalt or with the aldehyde in the organic stream, or the oxygen is considered trapped in small gas bubbles dispersed in the organic and/or water phase and which bubbles are considered impossible to ignite, the dilution is required as a safety measure to cover all possible scenarios, such as loss of flow of olefinic material feed and hence of liquid oxo product while the syngas keeps flowing; startup and shutdown conditions; process upsets, emergency shutdowns and emergency block-in procedures. In many of those scenarios, the oxygen could stay present for much longer, and the only gas present could be hydrogen and/or carbon monoxide, both of which have very wide flammability ranges in mixture with air.
In the process disclosed in DE-A-19939491, the flow of cobalt-containing water that circulates over the downstream three-phase settler and is recycled to the point of air injection could possibly be considered as a suitable continuous liquid phase in which to disperse the bubbles that contain the oxygen, and protect it from deflagration. However, once the three phases are separated in the settler downstream of the air injection point, the separate gas phase in the top of the settler is in continuous contact with the equipment wall. It can, therefore, in certain scenarios, still develop to an explosive mixture, and thus could possibly deflagrate upon ignition.
The separate gas phase in the top of the settler in DE-A-19939491 is relatively rich in nitrogen due to the nitrogen in the air that was injected. This poses the problem of disposal of this gas phase, typically by combustion, because the heating value of the gas is reduced by the nitrogen present.
Another concern may rise when the offgas from the oxidative decobalting step is mixed, further downstream, with offgases from other sources that contain other flammables. In such cases, the occurrence of gas mixtures inside their flammability range should be avoided.
The need to use such a large amount of nitrogen places considerable constraints on the volumetric efficiency of the process and furthermore results in difficulties in disposal of the offgas stream from the demetalling process since of itself it has such a low heating value.